Oil and gas hydrocarbons are naturally occurring in some subterranean formations. A subterranean formation containing oil or gas sometimes is referred to as a reservoir. A reservoir may be located under land or off shore. Oil and gas reservoirs may be located in the range of a few hundred feet (shallow reservoirs) to a few tens of thousands of feet (ultra-deep reservoirs). In order to produce oil or gas, a well is drilled into a reservoir or into another subterranean formation in fluid communication with a reservoir.
In order to produce oil or gas, a well is drilled into a subterranean formation, which can be a reservoir or adjacent to a reservoir. As used herein, a “well” includes at least one wellbore drilled into a subterranean formation, which may be a reservoir or adjacent to a reservoir. A wellbore can have vertical and horizontal portions, and it can be straight, curved, or branched. As used herein, the term “wellbore” refers to a wellbore itself, including any uncased, openhole portion of the wellbore. As used herein, a “well” also includes the interior of a wellbore and a near-wellbore region. A near-wellbore region is the subterranean material and rock of a subterranean formation surrounding the wellbore. The near-wellbore region is the region within about 100 feet of the wellbore. As used herein, “into a well” means and includes into any portion of the well, including into the wellbore or into a portion of a near-wellbore region.
Various types of treatments commonly are performed on a well or subterranean formation. For example, stimulation is a type of treatment performed on a well or subterranean formation to restore or enhance the productivity of oil and gas from the well or subterranean formation. Stimulation treatments fall into two main groups: hydraulic fracturing and matrix treatments. Fracturing treatments are performed above the fracture pressure of the subterranean formation to create or extend a highly-permeable flow path between the formation and the wellbore. Matrix stimulation treatments include acid, solvent, and chemical treatments to improve the permeability of the near-wellbore formation, enhancing the productivity of a well. Matrix stimulation treatments are performed below the facture pressure of the subterranean formation. Other types of treatments include, for example, controlling excessive water production, relative permeability modification, consolidating an incompetent subterranean formation, the stabilization of fines, and the remedial treatment of proppant to help prevent proppant flow back during production from a well.
A treatment typically involves introducing a treatment fluid into a well. As used herein, a “treatment fluid” is a fluid used to resolve a specific condition of a wellbore or subterranean formation. As used herein, a “treatment fluid” also means the specific composition of a fluid at the time the fluid is being introduced into a wellbore. A treatment fluid is adapted to be used to resolve a specific purpose, such as stimulation, isolation, or control of reservoir gas or water. The word “treatment” in the term “treatment fluid” does not necessarily imply any particular action by the fluid.
As used herein, a fluid can be homogeneous or heterogeneous. An example of a heterogeneous fluid is a dispersion. Further, a treatment fluid can include a gas for foaming the fluid. As used herein, an “aqueous” fluid is a fluid that is either a homogeneous solution comprising water or a heterogeneous fluid wherein the external phase comprises water. As used herein, an “alcohol” fluid is a fluid that is either a homogeneous solution comprising alcohol or a heterogeneous fluid wherein the external phase comprises alcohol, and further wherein the alcohol is selected from the group consisting of alcohols having fewer than three carbon atoms. A fluid can be an aqueous-alcohol fluid.
A common stimulation treatment is “hydraulic fracturing,” sometimes simply referred to as “fracturing.” A treatment fluid adapted for this purpose sometimes is referred to as a “fracturing fluid.” The fracturing fluid is pumped at a sufficiently high flow rate and pressure into the wellbore and into the subterranean formation to create or enhance a fracture in the subterranean formation. Creating a fracture means making a new fracture in the formation. Enhancing a fracture means enlarging a pre-existing fracture in the formation.
A newly-created or extended fracture will tend to close together after the pumping of the fracturing fluid is stopped. To prevent the fracture from closing, a material must be placed in the fracture to keep the fracture propped open. A material used for this purpose is referred to as a “proppant.”
The proppant is in the form of a solid particulate, which can be suspended in the fracturing fluid, carried downhole, and deposited in the fracture as a “proppant pack.” The proppant pack props the fracture in an open condition while allowing fluid flow through the permeability of the pack. A particulate for use as a proppant is selected based on the characteristics of size range, crush strength, and insolubility.
The proppant is an appropriate size to prop open the fracture and allow fluid to flow through the proppant pack, that is, in between and around the proppant making up the pack. Appropriate sizes of particulate for use as a proppant are typically in the range from about 8 to about 100 U.S. Standard Mesh. The proppant is sufficiently strong, that is, has a sufficient compressive or crush resistance, to prop the fracture open without being deformed or crushed by the closure stress of the fracture in the subterranean formation. Further, a suitable proppant should not dissolve in fluids commonly encountered in a well environment. Preferably, a material is selected that will not dissolve in water or crude oil. Suitable proppant materials include, but are not limited to, sand, ground nut shells or fruit pits, sintered bauxite, glass, plastics, ceramic materials, processed wood, resin coated sand or ground nut shells or fruit pits or other composites and any combination thereof in any proportion.
A problem with a fractured formation can be proppant flow back during production from the formation. Loss of proppant from a fracture is a leading cause of production decline. In addition, the flow back of proppant can damage production equipment.
Another common treatment is for the consolidation of an incompetent or unconsolidated subterranean formation. An incompetent formation is relatively ductile and tends to flow under stress rather than deform by brittle faulting or fracturing. An unconsolidated formation contains particulate matter capable of migrating with produced fluids out of the formation and into a wellbore. Unconsolidated portions of subterranean formations include those that contain loose particulates that are readily entrained by produced fluids and those wherein the particulates are bonded together with insufficient bond strength to withstand the forces produced by the production of fluids through the zones. The presence of particulate matter, such as sand, in produced fluids may be disadvantageous and undesirable in that such particulates may abrade pumping equipment and other producing equipment and may reduce the fluid production capabilities of the producing portions of the subterranean formation.
One method of controlling unconsolidated particulates involves placing a filtration bed of gravel near the wellbore in order to present a physical barrier to the transport of unconsolidated particulate matter with the production of hydrocarbons. Typically, such so-called “gravel packing operations” involve the pumping and placement of a quantity of a desired particulate into the unconsolidated formation adjacent to the wellbore. Such packs are time consuming and expensive to install.
Weakly consolidated formations also have been treated by creating fractures in the formations and depositing proppant in the fractures wherein the proppant is consolidated within the fractures into hard, permeable masses using a resin composition to reduce the migration of particulates. In some situations, the processes of fracturing and gravel packing are combined into a single treatment to stimulate hydrocarbon production while inhibiting particulate matter production with an annular gravel pack. Such treatments are referred to as “frac pack” operations.
Another method used to control formation particulates in unconsolidated formations involves consolidating a portion of a subterranean formation into a hard, permeable mass by applying a curable resin composition to the portion of the subterranean formation. An example of such a method includes preflushing the formation, applying a hardenable resin composition, applying a spacer fluid, applying an external catalyst to cause the resin to set, and applying an afterflush fluid to remove excess resin from the pore spaces of the zones. Such resin consolidation methods may be limited by the ability to place the resin through enough of the unconsolidated portion of the formation to control the particulates adequately. The compositions are often unable to achieve significant penetration or uniform penetration into the portion of the subterranean formation. Conditions such as variable formation permeability; formation damage in the near-wellbore area; debris along the wellbore, a perforation tunnel, or a fracture face; and, compaction zones along the wellbore, a perforation tunnel, or a fracture face may make uniform placement of resin compositions extremely difficult to achieve.
Water-soluble polymers have been used at high concentrations and cross-linked to build a 3-dimensional network. This results in plugging off flow channels, which can be used for water control. At lower concentrations, the materials are adsorbed onto the mineral surfaces resulting in changes in relative permeability of different fluids through the matrix. Most of the water-soluble polymers used for relative permeability modification tend to be produced back slowly. Often, ionic species are copolymerized into the water-soluble polymer, which makes the water-soluble polymer be attracted ionically to the mineral surfaces and helps decrease this wash-off tendency.